feat: Support to serialize ExceptionGroup

[fregataa]

callosum.rpc.message.ErrorMetadata now has sub_errors field to contain sub exceptions of ExceptionGroup. callosum.rpc.channel.Peer raises RPCUserError or RPCInternalError derived from the ErrorMetadata, which has exceptions field that has sub exceptions.
Peer side can access to sub exceptions by RPCUserError.exceptions or RPCInternalError.exceptions.